Characteristic and synthesis mechanism of MgB2 nanoparticles in solid-state reactive sintering
MgB2 nanoparticles were synthesized by a one-step reactive sintering method. The sample was heated from room temperature to 994 K, and then directly cooled down at a rate of 40 K/min. The results of X-ray diffraction indicate that MgB2 superconductors with high quality were successfully prepared, and only a few impurities were detected on it. The sample consisted of two distinguishable structures by the TEM observation: nanoparticles and single crystals, which is a result of nonequilibrium conversion under the direct cooling condition. The nanoparticles with a diameter of about 10-20 nm are considered to be at the onset for the formation of MgB2 phase. Transition temperature (Tc) and critical current density (Jc) of the compound structures were determined to be 38.5 K and 1.8 x 105 A cm-2 by means of SQUID measurement, which indicate good superconducting properties. The inter-diffusion and dislocation incorporation mechanisms for the formation and growth of MgB2 nanoparticles are also proposed, and the further growth will be accelerated due to an adequate holding time at 994 K. 2008 Elsevier B.V. All rights reserved.